The expectation in the hard disc drive industry is that areal density will increase at compound annual growth rate (CAGR) of >60% and an annual increase in data rate of greater than 40% for the foreseeable future. To reduce media noise and provide a sharper transition for longitudinal recording systems, a higher coercivity and lower magnetic thickness (Mrt) media is preferred. The magnetic thickness is the product of media's remnant magnetic moment (Mr) and its physical thickness (t). As a result of reducing the Mrt of the media, which reduces the media grain volume, the media may switch its magnetization due to thermal excitation (the super-paramagnetic effect). Further, demagnetization fields are higher at higher linear density. This higher demagnetization field makes the onset of the super-paramagnetic effect even faster by decreasing the energy barrier between the two states of magnetization for the grains.
Perpendicular recording offers an advantage because the transition width is defined by the gradient of the head field, which should be intrinsically larger than the gradient for a corresponding longitudinal head. Because of near perfect orientation of the perpendicular media, and the fact that the demagnetization field stabilizes transitions when they are brought closer together, perpendicular media can provide lower media noise. Further, perpendicular media is relatively thick compared to longitudinal media, which should improve the thermal stability. In principle, perpendicular recording can offer a gain factor of 4 to 5 over longitudinal recording for the same criteria of thermal stability and signal-to-noise ratio (SNR).
Perpendicular recording heads include a write pole and a return pole which are coupled by a yoke or pedestal. Electric current in a coil positioned around the yoke or pedestal creates a magnetic field in the poles that is used to affect the magnetization of an associated magnetic storage medium. In conventional perpendicular heads there is a significant magnetic field under the return pole. The field under the return pole has to be minimized so that it is significantly below the nucleation field (Hn) of the media to avoid corrupting data on neighboring tracks. The field under the return pole will effectively lower the energy barrier between the two magnetic states, requiring a higher thermal stability factor to avoid possible corruption of data.
By increasing the thickness of the return pole, the field under the return pole can be reduced. However, there is still a large field at the trailing end of the return pole. In addition, a thicker return pole will deteriorate the high frequency performance for the writer. Eddy currents generated in thicker films will limit the high frequency performance of the writer.
Thus there is a need for a perpendicular magnetic recording head that has a reduced field under the return pole and minimal eddy current losses.